Active implantable medical devices (AIMDs) find applicability in neurostimulation systems that deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. AIMDs are typically implanted within a tissue pocket of the patient, and connected to neurostimulation leads that are implanted at a target stimulation site remote from the tissue pocket.
AIMDs that are used in neurostimulation systems take the form of two general types: fully implanted and radio-frequency (RF)-controlled. The fully implanted AIMD contains the control circuitry, as well as a power supply, e.g., a battery, all within an implantable pulse generator (IPG) connected to one or more leads with one or more electrodes for stimulating tissue, so that once programmed and turned on, the IPG can operate independently of external hardware. The IPG is turned on and off and programmed to generate the desired stimulation pulses from an external programming device using transcutaneous electromagnetic links. In contrast, the RF-controlled AIMD includes an external transmitter inductively coupled via an electromagnetic link to an implanted receiver-stimulator connected to one or more leads with one or more electrodes for stimulating tissue. The power source, e.g., a battery, for powering the implanted receiver, as well as control circuitry to command the receiver-stimulator, is contained in the external controller—a hand-held sized device typically worn on the patient's belt or carried in a pocket. Data/power signals are transcutaneously coupled from a cable-connected transmission coil placed over the implanted receiver-stimulator. The implanted receiver-stimulator receives the signal and generates the stimulation.
AIMDs typically incorporate a sealing enclosure or case (commonly referred to as a “can”) that contacts tissue when implanted within the patient. This enclosure is constructed from a biocompatible material, and typically a metallic material, such as titanium. The interior components contained within the case are typically electronic circuits designed for processes, such as physiological signal sensing, diagnosis, data storage, therapy delivery, and telemetry. The case serves to isolate the interior components, which are typically not biocompatible, from the biological environment. In some AIMDs, the case also serves as a common or return electrode that allows sensing and delivery of stimulation energy. This practice is commonly referred to as “monopolar” or “unipolar” sensing or therapy.
AIMDs are routinely implanted in patients who are in need of Magnetic Resonance Imaging (MRI). Thus, when designing implantable neurostimulation systems, consideration must be given to the possibility that the patient in which AIMD is implanted may be subjected to EMI generated by MRI scanners, which may potentially cause damage to the AIMD, as well as discomfort to the patient. In particular, in MRI, spatial encoding relies on successively applying magnetic field gradients. The magnetic field strength is a function of position and time with the application of gradient fields throughout the imaging process. Gradient fields typically switch gradient coils (or magnets) ON and OFF thousands of times in the acquisition of a single image in the presence of a large static magnetic field. Present-day MRI scanners can have maximum gradient strengths of 100 mT/m and fast switching times (slew rates) of 150 mT/m/ms, which can result in induced voltages with frequency content comparable to stimulation therapy frequencies. Typical MRI scanners create gradient fields in the range of 100 Hz to 30 KHz, and Radio Frequency (RF) fields of 64 MHz for a 1.5 Tesla scanner and 128 MHz for a 3 Tesla scanner.
To a certain extent, the AIMD case provides protection against electromagnetic interference (EMI) from environmental sources and medical diagnostic tools, such as MRI scanners. However, the strength of the gradient magnetic field may be high enough to induce voltages (5-10 Volts depending on the orientation of the lead inside the body with respect to the MRI scanner) on to the stimulation lead(s), which in turn, are seen by the AIMD electronics. If these induced voltages are higher than the voltage supply rails of the AIMD electronics, there could exist paths within the AIMD that could induce current through the electrodes on the stimulation lead(s), which in turn, could cause unwanted stimulation to the patient due to the similar frequency band, between the MRI-generated gradient field and intended stimulation energy frequencies for therapy, as well as potentially damaging the electronics within the AIMD. To elaborate further, the gradient (magnetic) field may induce electrical energy within the wires of the stimulation lead(s), which may be conveyed into the circuitry of the AIMD and then out to the electrodes of the stimulation leads via the passive charge recovery switches. For example, in a conventional neurostimulation system, an induced voltage at the connector of the AIMD that is higher than AIMD battery voltage (˜4-5V), may induce such unwanted stimulation currents. RF energy generated by the MRI scanner may induce electrical currents of even higher voltages within the AIMD.
In some embodiments, the induced RF electrical current is shunted to the AIMD case to protect AIMD internal components. However, the induced electrical current may be collected in an additive fashion, and result in significant RF current flow from the AIMD case into the tissue pocket surrounding the AIMD case. This, in turn, results in heating of the tissue, with the potential for patient discomfort or even tissue damage. For neurostimulation systems that use more than one lead electrode connection in the AIMD, the amount of impinging RF current from the leads may increase as the number of lead electrodes increases, provided that the RF current arrives at the AIMD with similar phases from the various lead electrodes. This potentially exacerbates this heating phenomenon for implanted neurostimulation systems with a large number of lead electrodes (e.g., multi-lead AIMDs having eight or more electrodes per lead).
There, thus, remains a need to prevent RF current from being conveyed to the case electrically connected to the internal electronics contained within an AIMD.